Generally, brushless DC (BLDC) motors are designed so that a rotor is made up of a permanent magnet and a stator is made up of an armature with a coil wound around a core. The BLDC motors are classified into a sine-wave current driving type and a square-wave current driving type according to the wave profile of current supplied to the armature. A BLDC motor is also called a permanent magnet motor, and has a broad application as a variable speed driving unit in the high performance driving field, because the BLDC motor has a counter-electromotive force wave profile of a trapezoidal shape, as well as a light weight, a compact size, a high efficiency, a small inertia and a simple driving circuit as compared with an induction motor having the same output power.
However, a BLDC motor generates pulsating or ripple torque and a resultant mechanical vibration due to cogging torque together with driving torque for rotation, overlapping between phases, a spatial harmonic wave or the like, which lead to a reduced efficiency. Here, cogging torque is generated by interaction between stator slots and rotor magnets. Cogging torque can be significantly reduced by skewing the stator slots or rotor magnets by a pitch of one slot. Additionally, the pulsating or ripple components caused by mutual torque generated at the regions where torques for each phase are overlapped can be constrained by exciting a stator current sophisticatedly.
Typical BLDC motors have a plurality of windings, which function as an electric circuit, inserted into a stator and/or a rotor. According to the type of winding, BLDC motors are classified into a concentrated winding type, in which independent coils are wound around each tooth formed on the steel core, and a distributed winding type in which a plurality of windings are distributed into the corresponding teeth to form one phase. Of them, the concentrated winding type has been more widely used, because the coil winding work is carried out on the outside and the windings are then inserted around each tooth, thus it is possible to accomplish easier automation than the distributed winding type.
In addition, the conventional multiphase BLDC motors of the concentrated winding type require current torque to be highly generated through a high input current for high-speed operation. Moreover, the conventional multiphase BLDC motors of the concentrated winding type that are constructed in series connection are designed so that coils constituting individual phases are connected in series. Therefore, there are drawbacks in that all BLDC motors have a high resistance value, thus limiting the quantity of input current to a lower level, which makes it more difficult to generate high torque and to operate at a high speed.
FIG. 1A is a schematic view depicting series connection of a BLDC motor having an outer rotor configuration according to the prior art FIG. 1B is a schematic view depicting series connection of a BLDC motor having an inner rotor configuration according to the prior art, and FIG. 1C shows an equivalent circuit of an A-phase winding in case of the series connection as in FIGS. 1A and 1B.
In a typical BLDC motor, the rotor is made up of a permanent magnet, and the stator is designed so that a coil is wound around a continuous arrangement of teeth and slots. Here, when the rotor is arranged on the outside of the stator, it is called an outer rotor structure. And, when the rotor is arranged on the inside of the stator, it is called an inner rotor structure.
Referring to FIGS. 1A and 1B, the stator 12 consists of nine teeth 13 and nine slots 14. The nine teeth 13 are wound, as in the concentrated winding type of stator, in such a manner that a coil 15 is wound around each three teeth in a sequence of A phase, B phase and C phase, each of which is connected in series. In this series connection, its equivalent circuit is configured as shown in FIG. 1C so that three resistance components R1, R2 and R3 are connected with each other in series and resistance R—A is relatively low. Therefore, there is a difficulty in driving the BLDC motor at a high speed due to a restriction of driving current. Specifically, a resistance loss is generated in proportion to I2R, and thus a coil making up each phase is connected in series and has a high resistance value. Therefore, the multiphase BLDC motor of the concentrated winding type constructed in series connection makes it difficult to operate with a high efficiency due to a high resistance loss. In addition, the multiphase BLDC motor of the concentrated winding type constructed with series connections must be constructed for a plurality of coils making up each phase to be connected in series, so that coil winding work must be carried out after all the cores are completely assembled. For this reason, the conventional BLDC motor having a series connection is not suitable for an automation process, so that it has a low productivity. Moreover, even BLDC motors manufactured through the same process have different properties.
As shown in FIG. 2, a switching circuit for driving the stator with three phases, for example A-phase, B-phase and C-phase, requires four switching devices Q1 to Q4 per phase. Examples widely used for the switching devices Q1 to Q4 are power semiconductors, such as Integrated Gate Bipolar Transistors (IGBTs), Metal-Oxide Semiconductor Field Effect Transistors (MOSFETs), Field Effect Transistors (FETs) and so forth.
Referring to FIG. 2, in an H-bridge 202 for the A-phase, each of the switching devices Q1 to Q4 is controlled according to driving signals A1+, A1−, A2+ and A2−. In an H-bridge 203 for the B-phase, each of the switching devices Q1 to Q4 is controlled according to driving signals B1+, B1−, B2+ and B2−. In an H-bridge 204 for the C-phase, each of the switching devices Q1 to Q4 is controlled according to driving signals C1+, C1−, C2+ and C2−.
In the BLDC motor constructed as the foregoing, gate driving signals for controlling the on/off state of each switching device Q1 to Q4 are generated. When the H-bridges are operated, Pulse Width Modulation (PWM) signals are applied to two of the four switching devices so as to turn on/off two switching devices in an alternate manner with respect to each other. In other words, by setting the driving signals A1+ and A1− in the N-pole position of the rotor to be in a high state at the same time and then turning the first and fourth switching devices Q1 and Q4 on, the current path in the H-bridges runs in a counterclockwise cross direction. In contrast, by setting the driving signals A2+ and A2− in the S-pole position of the rotor to be in a high state at the same time and then turning the second and third switching devices Q2 and Q3 on, the current path in the H-bridges runs in a clockwise cross direction. A dead time is set for not maintaining the driving signals A1+ and A1− and the driving signals A2+ and A2− in a high state at the same time.
The switching circuit needs four switching devices so as to drive one phase. Therefore, the BLDC motor needs 4*K number of power switching devices to be driven for a certain magnitude and direction of phase current, thus increasing production costs.